As data transmission amounts between server and CPUs have increased in recent years, the handling of data through transmission of electrical signals using conventional Cu wires are approaching the limit. In order to resolve this bottleneck, optical interconnection, that is to say, data transmission through optical signals is required. Furthermore, from the points of view of reduction in power consumption and reduction in the area of devices, elements formed by integrating optical components such as optical transmitters, optical modulators and optical receivers that are required for optical transmission/reception on a Si substrate become necessary.
Meanwhile, optical components integrated on a Si substrate are connected through optical fibers and, therefore, it is preferable to use the wavelength of 1.55 μm, with which loss is low in optical fibers, as a transmission wavelength band. Therefore, it is preferable to use Ge, which has an absorption end in the vicinity of 1.55 μm, as an absorbing layer of a photodetector to be used for optical transmission with a band of the wavelength of 1.55 μm.
Meanwhile, wavelength division multiplexing (WDM) transmission is required as the data transmission amount increases. In order to implement WDM transmission, Ge photodetectors having a high response sensitivity in a broad region of which the wavelengths are longer than 1.55 μm are necessary.
In general, when the temperature is cooled to room temperature while Ge is growing on a Si substrate, the Ge epitaxial layer is subject to tensile strain in the directions within the plane of the substrate due to the difference between the coefficients of thermal expansion of Si and Ge. As a result, the absorption end of Ge on the Si substrate has a longer wavelength as compared to Ge layers in a bulk state, as has been reported (see Non-Patent Literature 1). This works advantageously from the point of view of expansion of the wavelength band of the photodetectors.
Meanwhile, the element capacitance of photodetectors is required to be lowered from the point of view of increase in the high-speed response properties and, thus, it is necessary to make the element area smaller (make the element width narrower). In addition, from the point of view of increase in the response sensitivity properties, it is required to prevent photocarriers generated in the depletion layer from recombining or becoming trapped by lattice defects while drifting. In order to do so, it is likewise necessary to make the element area smaller (make the element width narrower).
Thus, it has been reported that a photodetector having Ge on a Si substrate as an absorbing layer is processed as a mesa element having a width of approximately of several μm (see Non-Patent Literature 2). FIG. 20 is a schematic cross sectional diagram illustrating a conventional photodetector having Ge as an absorbing layer where the photodetector is formed using an SOI substrate. A Si layer 83 provided on top of a Si substrate 81 with a BOX layer 82 in between is processed to form a p type Si mesa portion 84 and p type Si slab portions 85 on the two sides thereof. At this time, a waveguide in stripe form is formed so as to be connected to the p type Si mesa portion 84 through a tapered portion, though this is not shown.
A non-doped Ge layer is formed on top of this p type Si mesa portion 84 through selective growth and n type impurities are implanted into the surface thereof so as to provide an n++ type Ge contact layer 87, where the portion into which the impurities have not been introduced is an i type Ge light absorbing layer 86. Meanwhile, p type impurities are implanted into portions of the p type Si slab portions 85 so as to form p++ type Si contact portions 88.
Next, an oxide film 89 that becomes the upper clad layer of the waveguide in stripe form is formed, plugs 90 and 91 are formed and, then, an n side electrode 92 and p side electrodes 93 are formed. Light that propagates through the waveguide in stripe form passes through the p type Si mesa portion so as to reach, and be absorbed by, the i type Ge light absorbing layer 86 through evanescent coupling.